1. Field of the Invention
The present invention relates to a control technique for an internal combustion engine. In particular, the invention relates to a knock prevention technique for an internal combustion engine having a variable compression ratio mechanism and a variable valve timing mechanism.
2. Background Art
There are known conventional internal combustion engines having both a variable compression ratio mechanism and a variable valve timing mechanism as described in, for example, Japanese Patent Laid-open No. 2002-285876 (hereinafter referred to as “Patent Document 1”). The variable compression ratio mechanism mechanically changes the compression ratio (mechanical compression ratio). The compression ratio is the ratio of the volume existing within the cylinder when the piston is at the bottom dead center to the clearance-space volume existing therein when the piston is at the top dead center. Various propositions have so far been made concerning the configuration of the variable compression ratio mechanism. The configuration disclosed in Patent Document 1 changes the piston stroke. Those disclosed in Japanese Patent Laid-open No. Hei 2-40056 (hereinafter referred to as “Patent Document 2”) and Japanese Patent Publication No. Hei 6-92746 (hereinafter referred to as “Patent Document 3”) comprise a dedicated piston for changing the top clearance-space volume in the combustion chamber. The variable valve timing mechanism changes the valve timing of the intake valve. By changing the timing of closing the intake valve, it is possible to change the effective cylinder volume and consequently the effective compression ratio.
In Patent Document 1, a knock prevention technique is disclosed for an internal combustion engine having such a variable compression ratio mechanism and a variable valve timing mechanism as mentioned above. In the case of the knock prevention technique disclosed in Patent Document 1, if knocking is detected, the ignition timing is delayed to instantaneously prevent knocking. After that, the mechanical compression ratio and effective compression ratio are decreased while the ignition timing is being advanced toward the normal timing. That is, it can be said that the knock prevention technique disclosed in Patent Document 1 is a combination of widely-known prior art methods: delaying the ignition timing; decreasing the compression ratio by a variable compression ratio mechanism; and decreasing the effective compression ratio by a variable valve timing mechanism.
In a typical internal combustion engine, the ignition timing is set to the MBT (minimum advance for the best torque) to maximize the combustion efficiency. If the ignition timing is later than the MBT, the decreased combustion efficiency lowers the mileage and deteriorates the exhaust emission although the decreased combustion pressure can prevent knocking. In the above-mentioned prior art technique, although delaying the ignition timing to prevent knocking is instantaneous, the mileage is lowered at least while the ignition timing remains delayed.